I agree with koti; the important thing is to get them asking questions and then, even more importantly, get them to think about how they could answer the questions. Including the importance of doing a rough calculation to check the validity of an idea (even if you have to guess at the numbers involved - e.g. Fermi estimates).
In terms of basic physics concepts that would (at the right time) equip them to do this, I think I would choose:
1. Newton's laws of motion (these can be summarised in one sentence, so I am not counting it as 3 things!)
2. The fact we can separate out vector components of force, motion, etc. -- this amazed me when I learnt it and it still seems remarkable!
3. Newton's law of gravitation
These three allow you to solve quite a range of real world problems and discover surprising facts (like, all masses fall at the same rate).
4. The fact that some properties are conserved and that this is related to fundamental symmetries (and point out that this was proved by a woman, as science so often seems dominated by male figures)
5. Point out that all the above are only approximations and that they will find out that the real world is actually more complex and more interesting.